The present application relates generally to fiber optic communications and, more particularly, to optical communications using silicon photonic chips having optical modulators using silicon interfaced with passive planar lightwave circuits.
In the past few decades, the speed of electronic processing, powered by increasing levels of integrations and smaller gate geometries has overwhelmed the ability of these same silicon integrated circuits to transmit and receive the information that they process. More and more electrical power and chip real-estate is devoted to driving the higher capacitance lines that carry signals off the integrated circuits. Thus the bottleneck in electronics is frequently the communication between chips, modules, or systems.
At the very longest length scales, telecommunication companies use multi-wavelength communication down a single optical fiber to pack more than a hundred channels, each modulated using various techniques to transport information for thousands of kilometers. The optical line cards and transport systems are complex, large, and expensive, justified by the need for bandwidth efficiency in the very long links that they serve. Currently at the shorter distance scales of a few hundred meters to a few kilometers, the same multi-wavelength approach is used, albeit with a smaller number of channels and simple on-off (NRZ) modulation with more compact transceivers and at lower costs. In both types of multi-wavelength communications, laser sources, usually in Indium Phosphide materials systems generate the light, and the data is then imposed on the signal. In the simplest case, the drive current to the laser is changed to vary the optical output intensity, while in more complex systems a separate modulator receives a continuous optical signal from the laser and acts to vary the intensity or the phase of the light that passes through it. The latter is of course more expensive and complicated, but can be more precise, as a separate modulator can more controllably vary the properties of the light.
Recently there has been a great deal of excitement in the prospect of using silicon as the material for the modulator. The idea is that the industrial infra-structure that allows the fabrication of complex electronic integrated circuits can be leveraged to fabricate the modulators. Such technology can be useful at all length scales, from complex modulators on the silicon that can create intensity and phase modulation for efficient packing of wavelength channels in very long links (for example DQPSK modulation—Differential Quad Phase Shift Keying, used in long haul links) to simple on-off modulation to code ones and zeros (NRZ-non return to zero) in shorter links.
Perhaps the most significant issue with silicon photonics is that silicon as a material, unlike Indium Phosphide, does not possess a direct bandgap. By that we mean that electrons and holes of the lowest energy have different momentum states, and therefore cannot combine directly to generate light. In a forward biased silicon pn junction, the carriers recombine non-radiatively and thus one cannot make LEDs or lasers in silicon. Generally there have been three workarounds for this problem. The first is obviously to have the light off the chip, so a separate indium phosphide laser generates the light and the light is then coupled to the silicon chip where it is modulated and then sent out. The challenge here is of course the complexity of getting the light on and off the silicon chip, especially if multiple wavelengths or multiple sources of light are needed. The second more ambitious way is to try to incorporate the direct gap indium phosphide material on the silicon. The different lattice constant, chemistry, and processing requirements of the indium phosphide make it difficult to fabricate efficient lasers this way. Furthermore, it is impossible to test or burn-in the laser prior to assembly and the relatively poor yield of the lasers increases the cost of the entire assembly. Perhaps the ultimate solution is to try to make the silicon direct gap by adding impurities or changing the crystal through physical deformation. Needless to say, this is very challenging.
A second related issue with silicon photonics is the challenge of coupling light in and out of the chip. Even if the light-source can be integrated into the silicon, one still requires the light to exit the chip and enter an optical fiber. Silicon modulators typically use extremely small and high contrast waveguides. The core is usually made of silicon that is a few hundred nanometers in scale, and the cladding is typically silicon dioxide with a very low refractive index compared to the silicon core (1.46 vs 3.6). Thus the light is highly concentrated in a very tight waveguide. The high contrast has the advantage of being able to make tight waveguide turns, the light paths almost having the geometries of electrical wires, but also has the disadvantage of being completely mismatched to a mode in a glass optical fiber, where the contrast is typically much less than 1% between the core and the cladding. Grating couplers are frequently used to help with the alignment, but grating couplers generally work only at one wavelength and therefore limit the coupling to a single channel per port.
In current architectures where fiber optics is used to connect electronic switches, the optics is separate and usually in the form of a transceiver that is plugged in to the faceplate of the unit. Typical switches used in datacenters can have tens or even hundreds of optical transceivers that populate the front plate of the unit. One advantage of this is that the customer can easily replace faulty transceiver units at the front panel. The switch itself generally does not need to be removed or sent back to the supplier for repair in the event of faulty transceivers. However, there are many penalties with this approach. First it is difficult to cool the transceivers in the front panel. It would be much easier if the modules were mounted on a board of the switch. A second issue is that high speed signals have to travel from a switch chip, somewhere on the board, all the way to the front panel. There is frequently equalization that has to occur both on the board and also in the transceiver to compensate for distortion and electrical signal loss as the high speed data patterns travel the distance from the source into the transceiver and to the optical module.